The present invention relates to a parking brake system with transmission interlock and pertains particularly to a control system that automatically vents transmission control pressure when the parking brake is engaged.
Large heavy-duty vehicles, such as earth-working vehicles, are normally provided with parking brake systems that automatically engage when the vehicle engine is shut down and transmission control pressure is permitted to leak down. These systems are normally constructed such as to combine the parking brake with the service brake system. The parking brake is normally spring-applied and fluid pressure released. Thus the parking brake is normally engaged and remains engaged until sufficient control pressure is developed within the system to disengage the parking brakes. One problem with such vehicles, however, is that the engine capacity is such as to be able to move the vehicle with the parking brakes engaged. Such movement, however, could severely damage the brake system.
Most systems are provided with some form of warning means such as buzzers or flashing lights to warn the operator that the brakes are still engaged. However, should such a warning system fail, the operator may forget to disengage the parking brake and thus drive the vehicle off and damage the parking brakes and consequently the service brakes such as to make them essentially non-workable
There are times, however, when it would be advantageous to be able to move the vehicle such as to a shop or other service area if brake pressure is lost. This cannot generally be accomplished with known prior art systems.